Wavelength-converted laser systems often use nonlinear optical crystals, such as lithium borate (LBO) to generate a visible wavelength output beam from an infrared input beam generated by a source laser. In second harmonic generation (SHG), for example, a non-linear process taking place in the crystal combines two photons of infrared input radiation to produce a photon of visible output radiation having twice the frequency of the input infrared radiation. In third harmonic generation (THG), second harmonic generation is combined with an additional nonlinear optical crystal that is phase matched to combine a photon of the SHG output with a photon of the infrared input to produce third harmonic generation (THG) output having three times the frequency of the infrared input radiation.
Nonlinear crystals such as LBO are often characterized by an anisotropic refractive index, and this has an effect on the shape of the wavelength-converted output beam. An anisotropic refractive index means that the index of refraction depends on the direction of propagation and polarization of radiation in the crystal. If an unpolarized beam is launched into such a crystal, it will typically exhibit double refraction: the beam will split into two polarized beams that are not collinear but whose directions of propagation differ by an angle called the walk-off angle.
When a crystal with an anisotropic refractive index such as LBO is used for second harmonic generation, there will be walk-off among the beams. Either the output beam travels at a walk-off angle with respect to the input beam, or else the input beam itself undergoes double refraction, breaking up into two beams, one of which propagates at the walk-off angle. Because of the walk-off, the output beam is distorted and ends up with a different shape than the input beam. Typically the input beam is circular, and the output beam is elliptical.
The problem of beam shape distortion is further compounded in third harmonic generation. Three beams propagate in the THG crystal: the input fundamental beam, the second harmonic beam, and the third harmonic output beam. One of the beams travels at a walk-off angle with respect to the other two, and this walk-off distorts the THG output beam. Because the second harmonic beam is typically noncircular already, the third harmonic beam is generally even more noncircular than the second harmonic beam.
However, in many practical applications that use nonlinear optical wavelength conversion it is desirable for the output beam to have a circular cross-section. One solution is to use two cylindrical lenses to image the wavelength-converted output beam to the desired location and shape. Unfortunately, an optical relay using two cylindrical lenses is extremely sensitive to slight rotations of the lenses. Rotational misalignments on the order of milli-radians can introduce severe aberrations into the beam. Furthermore, if the output beam is ultra-violet, it is often undesirable to use any beam-shaping optics at all, since ultra-violet light quickly degrades most bulk materials and coatings that are put in its path.
It is within this context that embodiments of the present invention arise.